The present invention relates generally to methods of cleaning exhaust gas, and in particular to a new and useful method for regenerating a catalyst used to remove nitrogen oxides from exhaust gas produced by the burning of coal.
Selective catalytic reduction (SCR) technology is used worldwide to control NOx emissions from combustion sources at higher temperatures (550-750 degrees F.). High temperature SCR technology has been used in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the late 1990's. The function of the SCR system is to react NOx with ammonia (NH3) and oxygen in the presence of a catalyst to form molecular nitrogen and water.
As shown in FIG. 1, SCR systems are located in a stream of flowing flue gas 15. Ammonia is injected into the hot flue gas upstream of the selective catalytic reduction reactor 20 by an ammonia injection system 10, such as an ammonia injection grid. Known systems for injecting ammonia upstream of an SCR catalyst are described in U.S. Pat. Nos. 5,380,499, 5,437,851 and 6,887,435, all assigned to The Babcock & Wilcox Company at issue, and which are hereby incorporated by reference as though fully set forth herein. The flue gas, with the ammonia, passes across the surface of the SCR catalyst 30, which is arranged in several layers within reactor 20. Industrial scale selective catalytic reduction reactors have been designed to operate principally in the temperature range of 500 degrees F. to 900 degrees F., but most often in the range of 550 degrees F. to 750 degrees F. Ash entrained in the flue gas may deposit on catalyst 30, and reactor 20 may include catalyst cleaning devices 50, such as sootblowers and/or sonic horns.
Additional details of the characteristics of SCR systems are available in Chapter 34 of Steam/Its Generation and Use, 41st Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., © 2005, the text of which is hereby incorporated by reference as though fully set forth herein.
Catalysts 30 are typically modestly noble metals such as vanadium, titanium, molybdenum and tungsten and a variety of their oxides. These catalysts are generally preferred because they exhibit good resistance to sulfur poisoning.
Chemical poisoning of SCR catalysts occurs in all types of coal combustion flue gases. SCR catalysts are chemically deactivated by catalyst poisons, which are contained in the coal combustion flue gases or fly ash in the form of heavy metals such as mercury, arsenic, thallium, etc. This “chemical poisoning” results from the reaction of SCR active components such as W, V, and Mo, with, for example, oxides of the heavy metals compounds and/or phosphate Reversing chemical poisoning and regenerating SCR catalyst typically requires complicated, multi-step procedures. For example, U.S. Pat. No. 6,596,661 describes a 4-step procedure to regenerate a chemically poisoned catalyst. This procedure involves taking the SCR off-line (by means of a by-pass) and contacting the catalyst with 1) a reducing agent and 2) washing the catalyst with a polyfunctional complex forming agent such as hydrocarboxylic acid. Steps 1 and 2 eliminate the chemical bonds between poisons and the SCR active components, and redistribute the remaining active components. In step 3, the catalyst is contacted with a solution or a suspension of active components (such as V, W, . . . ) in the polyfunctional agent solution in order to restore the original activity of the SCR catalyst. In the final step (step 4) the catalyst is dried by air at about 160 degrees F. This regeneration process is complicated, time-consuming and requires the SCR to be taken off-line.
Fuel cost issues, as well as strict SO2 and SO3 emissions limits, have resulted in a significant increase in the number of US utilities burning low sulfur coal from the Powder River Basin (PRB) of Wyoming and Montana. Many utilities burning PRB coal are now confronted with the necessity of installing SCR units to meet strict NOx emission limits. There are a number of uncertainties regarding SCR activity performance in PRB coal combustion systems. Unexpected and accelerated deactivation of SCR catalysts exposed to PRB coal combustion flue gas has been observed.
Rigby et al., of Siemens KPW, in their paper “SCR Catalyst Design Issues and Operating Experience: Coals with High Arsenic Concentrations and Coal from the Powder River Basin” (in the Proceedings of 2000 International Joint Power Generation Conference, Miami Beach, Fla., Jul. 23-26, 2000, IPJGC2000-15067) have provided a comprehensive review of the influential parameters in PRB coal combustion that can lead to an accelerated deactivation of SCR catalyst, the text of which is hereby incorporated by reference as though fully set forth herein. The authors concluded that the main deactivation mechanism for SCR catalysts exposed to PRB coal combustion flue gases is most likely the formation of a dense, calcium sulfate (CaSO4) layer on the surface of the catalyst. This layer blocks the entrance of the flue gas to the pores of the catalyst, thus masking the active sites of the catalyst. The authors also concluded that the presence of large amounts of free calcium oxide (CaO) is the essential factor in the CaSO4 formation mechanism. FIG. 2 is a schematic diagram from the paper illustrating the calcium sulfate masking of an SCR catalyst.
The Rigby et al. authors proposed the following mechanism for the formation of a calcium sulfate surface coating on SCR catalysts in PRB applications:
(1) Free CaO (in fly ash) is deposited onto catalyst surface
(2) SO2 (in exhaust gas)→SO3 (on catalyst surface)
(3) Free CaO (on catalyst surface)+SO3 (g)→CaSO4 (calcium sulfate coating)
It is apparent that an economical and easy to implement method of reactivating a catalyst deactivated due to masking by a calcium sulfate layer would be welcomed by industry.